Some considerations on the radioactive waste assay system

Transporting and storing the containers of radioactive waste requires a characterization of radionuclide content in order to verify the complying with transportation and waste management requirements. Since a low level of detection is desired, the gamma spectroscopy is often the only viable method for the content assay. This method is based on the gamma spectrum analysis and requires a suitable efficiency calibration for bulk containers. The present study is focused on the efficiency calibration of the gamma spectroscopy system for a drum waste assay developed in IFIN-HH (Horia Hulubei National Institute for Physics and Nuclear Engineering). In order to minimize the effect of non-homogeneities (matrix density, radioactive source distribution, etc) and to obtain a complete covering of the drum surface, the system is provided with an electromechanical device which enables the simultaneously rotation and horizontal translation of the drum. A double translation (forward and backward) is performed to obtain the best possible accuracy, independent of the distribution of radioactive material in the drum. A lead collimated Ortec HPGe detector of 20% relative efficiency connected to 8192 channel MCA was used. The radioactive waste is generally conditioned in 220 l stainless steel containers. For the efficiency calibration we used a similar drum filled with Portland cement and a linear source of 152 Eu (Λ = 91.02 ± 4.55 MBq) produced by the Radionuclide Metrology Group from the Radioisotope Department, IFIN-HH. The calibration drum is provided with 7 hollow tubes of 20 mm diameter placed at different distances from the center of the drum. The linear calibration source was introduced successively in the 7 tubes and the gamma spectra were recorded, while the drum was rotated and simultaneously translated. Using ASPERA 8k software (developed by IFIN-HH) the gamma spectra were analyzed and the average values of the net peak area for each of the selected energies (121.8 keV, 344.2 keV, 778.9 keV, 1112 keV and 1408 keV) and for each radial source position were obtained. These values were integrated over the drum volume and the total response of the detector was obtained. The detection efficiency was calculated for each of the selected energies. The calibration curves for several values of the cylinder radius, considering an approximately uniformly distributed source, are shown. The minimum detectable activity (MDA) for the given geometry, namely, a collimated HPGe (20% relative efficiency), a 220 l cement filled drum and 10 min. measuring time is approximately 45 Bq/kg for 137 Cs and 60 Co. These values of MDA are considered as detection limits of the system. By increasing the measuring time, smaller values for MDA could be obtained. The total measurement uncertainty ranges from - 70% to + 40% including the worst cases where the total activity is concentrated in one point located in center-bottom of the drum for gamma ray energy less than 500 keV. (authors)